Do they like museums? Galleries? Historic houses? Shopping?
If they are a little bit interested in history, then I would take them to the Tower of London (there are great tours) and a boat trip from outside to Greenwich to see the Meridian line and the view, then the DLR (driverless trains) back through the Docklands to central London. If that doesn't sound like their bag, most kids really get off on the London Dungeon which is close to London Bridge station. Covent Garden is fun at that age - there's usually a pretty good juggler/magician/tumbler performing in the piazza and there are lots of shops.
The V&A museum is eclectic enough to have something for everyone and there is a hotel nearby where you can get a great afternoon tea cheaply (2-for-1, see below), it's also a short bus ride to Harrods if they want to buy some olive green tat
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Finally, a bit of a curveball, the Foundling Museum is wonderful, really moving and there is a great playground nearby which your son might enjoy.
BIG TIP: Look at the 2-for-1 Days Out website - if you buy your travel cards at a rail station when you arrive, then you will be entitled to lots of attractions on a 2-for-1 basis. You can either print off the vouchers at home or most railway stations have booklets that list a lot of the attractions and little tear out vouchers at the back.